Apparently there used to be three branches– the main line (still in use), the North Shore line, and the South Beach line. I think the sunken abandoned stations that I saw were on the South Beach line, around Mariners’ Harbor.

[…] The first air conditioned cars rolled into service in the late 1960s, about six decades after the subway opened. But because subway cars last so long, it took decades more for the new models to replace the old ones. […]

As a kid the fast-moving trains were cooler when the windows were open and the dank tunnel air raced through the dim, incandescent-lit cars. Wearing a stripped t-shirt and shorts (purchase by my mom from E.J Korvette’s) the wicker seats would leave their impression on the back of my bare legs.

Not only were there ceiling fans blowing hot air around, but all the windows were open, as well as the doors between the cars. It was part of the culture of traveling the subways. I don’t recall air conditioning until around 1970 when a new line of trains had air conditioning.